Transparent or pigmented powder coating materials with crosslinkers comprising hydroxyalkylamides and polyisocyanates containing uretdione groups

ABSTRACT

Transparent or pigmented powder coating materials with crosslinkers comprising hydroxyalkylamides and polyisocyanates containing uretdione groups.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to transparent or pigmented powdercoating materials with crosslinkers comprising hydroxyalkylamides andpolyisocyanates containing uretdione groups, which materials possessreduced susceptibility to pinholing.

[0003] 2. Discussion of the Background

[0004] Powder coating materials based on triglycidyl isocyanurate (TGIC)and acid-functional polyesters produce corrosion-resistant andweather-stable powder coatings. EP 0 536 085 describes, however, howexpensive processes or a relatively large and thus likewise expensivepurification effort are required in order to prepare the TGIC in solidform. Moreover, TGIC is classified by the European Community as acategory II mutagen, which is that it is regarded as a cause ofmutations, and as of May 31, 1998 has required that it be labeled as“toxic”.

[0005] Toxicologically unobjectionable and at the same time morereactive as well are β-hydroxyalkylamide crosslinkers. In U.S. Pat. Nos.4,076,917 and 4,101,606, β-hydroxyalkylamides are combined with polymershaving at least one carboxyl or anhydride function, in particular withpolyacrylates, to form powder coating materials. U.S. Pat. No. 4,988,767describes powder coating materials based on hydroxyalkylamides andacidic acrylate resins. EP 0 322 834 describes thermosetting powdercoating materials composed of β-hydroxyalkylamides and polyesterscontaining acid groups. These coatings with β-hydroxyalkylamidecrosslinker are highly weather-stable, very flexible, hard, andchemical-resistant. Especially at relatively high coat thicknesses,however, the water which is released on crosslinking leads to pinholing.

[0006] U.S. Pat. No. 4,889,890 describes hybrid powder coatingmaterials. The acidic polymers they contain are cured with polyepoxidessuch as, for example, polyglycidyl ethers of aromatic phenols. In orderto increase the crosslinking density, a β-hydroxyalkylamide is used.These hybrid powder coating materials afford good corrosion protectionbut are unsuited to use outdoors because of a lack of weatheringstability.

[0007] Thometzek et al. in 5th Nuremberg Congress, Congress Papers,Volume 1, 251-273 (1999) describe powder coating materials comprising ahydroxy-functional polyester resin, a hydroxyalkylamide, and abifunctional crosslinker which in addition to blocked isocyanate groupscontains carboxyl groups. These powder coating materials are suitablefor obtaining matte rather than glossy coatings.

SUMMARY OF THE INVENTION

[0008] Accordingly, one object of the present invention is to providepowder coating materials which give highly glossy, flexible,light-stable and weather-stable coatings of low susceptibility topinholing.

[0009] Briefly, this object and other objects of the present inventionas hereinafter will become more readily apparent can be attained by atransparent or pigmented powder coating material containing crosslinkerscomprising hydroxyalkylamides and polyisocyanates containing uretdionegroups, comprising:

[0010] a) from 20 to 99% by weight of at least one acidic polymer;

[0011] b) from 1 to 25% by weight of at least one β-hydroxyalkylamide;

[0012] c) from 0.05 to 10% by weight of at least one nonaromaticpolyisocyanate which contains uretdione groups and has an NCOfunctionality≧1;

[0013] d) from 0 to 50% by weight of at least one pigment and/orinorganic filler;

[0014] e) from 0 to 5% by weight of at least one adjuvant or auxiliary.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] It has surprisingly been found that coatings comprising acidicpolymers and β-hydroxyalkylamide crosslinkers, especially at relativelygreat coat thicknesses, are markedly less susceptible to pinholing ifpolyisocyanate crosslinkers containing uretdione groups are added as anadditional component to the coating formulation. Flexibility, gloss,reactivity, and weather stability remain at the traditional high levelof polyester/hydroxyalkylamide or polyurethane powder coating materials.

[0016] Suitable acidic polymers A) for the powder coating material ofthe present invention include polyester resins and acrylate resins.

[0017] The acidic polyesters possess a glass transition temperature (Tg)in the range from 30 to 85° C. and an acid number ranging from 20 to 80mg KOH/g. These acidic polyesters are prepared primarily by acondensation reaction of aliphatic polyols and/or cycloaliphatic polyolswith aliphatic and/or aromatic polycarboxylic acids and anhydrides.Polyesters of this kind are described, for example, in EP 0 322 807, DE198 50 970 and WO 95/01406.

[0018] The acidic acrylate resins possess an acid number in the rangefrom 10 to 250 mg KOH/g and a melting point of from 60 to 160° C. Theyare prepared primarily by copolymerizing a monomer mixture consistingof:

[0019] a) from 0 to 70 parts by weight of methyl (meth)acrylate,

[0020] b) from 0 to 60 parts by weight of (cyclo)alkyl esters of acrylicacid and/or methacrylic acid having 2 to 18 carbon atoms in the alkyl orcycloalkyl radical,

[0021] c) from 0 to 90 parts by weight of vinylaromatic compounds, and

[0022] d) from 0 to 60 parts by weight of olefinically unsaturatedcarboxylic acids, the sum of the parts by weight of components a) to d)being 100.

[0023] The monomers b) comprise (cyclo)alkyl esters of acrylic ormethacrylic acid having 2 to 18 carbon atoms in the (cyclo)alkylradical. Suitable examples of such ester monomers b) preferably includeethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate,n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl methacrylate,neopentyl methacrylate, isobornyl methacrylate,3,3,5-trimethylcyclohexyl methacrylate, stearyl methacrylate and thelike.

[0024] Suitable examples of monomers c) include styrene, vinyltoluene,ethylstyrene and the like. Suitable examples of unsaturated acidmonomers d) include acrylic and methacrylic acid, which are alsopreferred acids, as well as crotonic acid, itaconic acid, fumaric acid,maleic acid, citraconic acid and the like.

[0025] The copolymers may be prepared by copolymerizing the exemplifiedmonomers a) to d) in accordance with customary free-radicalpolymerization processes, such as solution, emulsion, bead or bulkpolymerization, for example.

[0026] The monomers are copolymerized at temperatures ranging from 60 to160° C., preferably from 80 to 150° C., in the presence of free-radicalinitiators and, if desired, molecular weight regulators.

[0027] The carboxyl-functional acrylate copolymers are prepared in inertsolvents. Examples of suitable solvents include aromatic compounds, suchas benzene, toluene, xylene; esters, such as ethyl acetate, butylacetate, hexyl acetate, heptyl acetate, methylglycol acetate,ethylglycol acetate, methoxypropyl acetate; ethers, such astetrahydrofuran, dioxane, diethylene glycol dimethyl ether; ketones,such as acetone, methyl ethyl ketone, methyl isobutyl ketone, methyln-amyl ketone, methyl isoamyl ketone; or any desired mixtures of suchsolvents.

[0028] The copolymers may be prepared continuously or batchwise. Atypical procedure is to meter the monomer mixture and the initiator intoa polymerization reactor continuously and at a uniform rate and at thesame time to continuously withdraw the corresponding amount of polymer.In this way it is possible, preferably, to prepare copolymers which arevirtually uniform in chemical terms. Chemically near-uniform copolymersmay also be prepared by running the reaction mixture at a constant rateinto a stirred vessel without removing the polymer.

[0029] Alternatively, a portion of the monomers, by way of example, maybe introduced as an initial charge in solvents of the stated type andthen the remaining monomers and auxiliaries, separately or together, maybe introduced into this initial charge at the reaction temperature. Ingeneral, the polymerization takes place under atmospheric pressure, butmay also be conducted at pressures of up to 25 bar. The initiators areused in amounts ranging from 0.05 to 15% by weight, based on the overallamount of the monomers.

[0030] Suitable initiators include the usual free-radical initiators,such as aliphatic azo compounds, of which examples includeazodiisobutyronitrile, azobis-2-methylvaleronitrile,1,1′-azobis-1-cyclohexanenitrile and 2,2′-azobis-isobutyric acid alkylesters; symmetrical diacyl peroxides, which include acetyl, propionyland butyryl peroxide, benzoyl peroxides substituted by bromine, nitro,methyl or methoxy groups, and lauryl peroxides; symmetricalperoxydicarbonates, e.g., tert-butylperbenzoate; hydroperoxides, such astert-butyl hydroperoxide and cumene hydroperoxide; dialkyl peroxides,such as dicumyl peroxide, tert-butyl cumyl peroxide and di-tertbutylperoxide. In order to regulate the molecular weight of the copolymers itis possible to employ customary regulators in the preparation. Examplesof such include mercaptopropionic acid, tert-dodecyl mercaptan,n-dodecyl mercaptan and diisopropylxanthogen disulfide. The regulatorsmay be added in amounts ranging from 0.1 to 10% by weight, based on theoverall amount of the monomers.

[0031] The copolymer solutions obtained after copolymerization can thenbe subjected, without further work-up, to evaporation ordevolatilization, by which the solvent is removed. For example, thesolution can be placed in a devolatilizing extruder or spray dryer at atemperature ranging from about 120 to 160° C. under a vacuum of from 100to 300 mbar. The copolymer product is then recovered.

[0032] Polyacrylates of this kind are described, for example, in U.S.Pat. No. 4,988,767. As polycarboxyl compounds it is of course alsopossible to use mixtures of two or more substances.

[0033] The β-hydroxyalkylamides B) are known and are described, forexample, in EP 0 957 082, U.S. Pat. No. 4,076,917 and U.S. Pat. No.4,101,606. Suitable β-hydroxyalkylamides include those having thefollowing structure:

[0034] where:

[0035] X is a chemical bond, hydrogen or a monovalent or polyvalentorganic group derived from saturated, unsaturated or aromatichydrocarbon groups and having 1-24 carbon atoms, or these radicalssubstituted by heteroatoms;

[0036] R¹ is hydrogen, an alkyl, alkenyl, aryl or aralkyl radical having1-24 carbon atoms, these radicals substituted by heteroatoms, or

[0037] wherein each of R² independently is identical or different, andis a radical selected from hydrogen, an alkyl, aryl, aralkyl or alkenylradical having 1-24 carbon atoms, or these radicals substituted byheteroatoms;

[0038] n is an integer from 1 to 10;

[0039] m is 0, 1 or 2; and

[0040] n+m≧2.

[0041] Preferred compounds used to prepare the powder coating materialsof the invention are prepared in accordance with the procedure disclosedin EP 0 957 082 as described on page 4 of the A2 text [0026].

[0042] The ratio of the carboxyl-containing polymers A) to theβ-hydroxyalkylamide compound B) is generally selected such that theratio of carboxyl groups to hydroxyl groups ranges from 0.6:1 to 1.6:1.

[0043] The nonaromatic polyisocyanates containing uretdione groups, C),and having an NCO functionality>1 are known. They are described, forexample, in DE 30 30 539, 44 06 444, DE 195 47 878, DE 196 33 218 andEP0 639 598.

[0044] The polyisocyanate c) comprises uretdiones of aliphatic,(cyclo)aliphatic and cycloaliphatic diisocyanates. Preferred uretdionesare derived from the diisocyanates isophorone diisocyanate (IPDI),hexamethylene diisocyanate (HDI), 2-methylpentyl diisocyanate (MPDI) and2,2,4(2,4,4)-trimethylhexamethylene diisocyanate (TMDI), which may beused alone or in mixtures. In general, the amount of component c) in thepowder coating materials of the invention ranges from 0.05 to 10% byweight, preferably from 0.05 to 7% by weight.

[0045] Polyols suitable for chain extension are described, for example,in DE 19 57 483, DE 21 05 777, DE 25 42 191 and DE 27 38 270. Preferredpolyols include ethylene glycol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 3-methyl-1,5-pentanediol,2,2,4(2,4,4)-trimethylhexanediol, neopentyl glycol hydroxypivalate ortrimethylolpropane. Linear or branched hydroxyl-containing polyesters orpolycaprolactones may also be used as chain extenders.

[0046] The polyisocyanate C) may carry, terminally, either free orirreversibly blocked NCO groups or hydroxyl groups. The compounds thatmay be used in principle for irreversible blocking of thepolyisocyanates include, for example, monohydric alcohols such asethanol, n-butanol or 2-ethylhexanol and primary or secondary monoaminessuch as n-propylamine, n-butylamine, n-hexylamine, dibutylamine ordicyclohexylamine, for example.

[0047] The ratio of the β-hydroxyalkylamide compound b) to thenonaromatic polyisocyanate containing uretdione groups c), is generallyselected such that the ratio of hydroxyl groups to isocyanate groupsranges from 0.5:1 to 2.0:1.

[0048] In order to prepare the ready-to-use powder coating materials,the acidic polymer a), the β-hydroxyalkylamide b), the nonaromaticpolyisocyanate containing uretdione groups, c), together, if desired,with pigments or fillers d) such as TiO₂ or barium sulfate, for example,and further the usual powder coatings adjuvants or auxiliaries e) suchas leveling agents such as polybutyl acrylate or degassing agents suchas benzoin or catalysts such as dibutyltin dilaurate (DBTL) or C₁-C₁₈zinc, aluminum or titanium carboxylate salts or aluminum oxides or zincoxides, for example, are mixed. All ingredients of the powder coatingmaterial are homogenized in a melt. This can be done in a suitableapparatus, such as treatable compounders, for example, but preferably byextrusion, in the course of which the temperature ought not to exceed anupper limit of 140° C. After cooling to room temperature and appropriatecomminution, the extruded mass is ground to a ready-to-spray powder.This powder may be applied to appropriate substrates by known processes,such as by means of electrostatic or tribostatic powder spraying or byfluidized bed sintering, with or without electrostatic assistance, forexample.

[0049] Following powder application, the coated work pieces are cured byheating at a temperature ranging from 140 to 220° C. for from 60 to 5minutes.

[0050] The prior art powder coating materials based on acidic polymersand β-hydroxyalkylamides have the disadvantage that they are susceptibleto pinholing, especially at relatively high coat thicknesses.

[0051] Surprisingly, the minor addition of polyisocyanate crosslinkerscontaining uretdione groups results in improved coating resistance tothis kind of surface defect. It is, therefore, possible to obtaingreater process reliability when coating. At the same time, it ispossible to utilize the known advantages of polyurethane crosslinking,such as excellent adhesion, flexibility, light stability and weatherstability, for example.

[0052] Having now generally described this invention, a furtherunderstanding can be obtained by reference to certain specific Exampleswhich are provided herein for purposes of illustration only and are notintended to be limiting unless otherwise specified.

EXAMPLES

[0053] 1 Raw Materials Used

[0054] 1.1 Acidic Polymer A)

[0055] The acidic polymer used was the polyester GRILESTA® P7617 (acidnumber 35 mg KOH/g, EMS-Inventa AG).

[0056] 1.2 β-Hydroxyalkylamide B)

[0057] The β-hydroxyalkylamide B) used was the polyester VESTAGON® EP-HA320 (OH number 668 mg KOH/g, Degussa-Huls AG).

[0058] 1.3 Blocked Polyisocyanate C)

[0059] Polyisocyanates C) used were C1) VESTAGON® BF 1540(polyisocyanate adduct containing uretdione groups, NCO number 15.9%,Degussa-Huls AG) and C2) VESTAGON® EP-BF 1310 (polyisocyanate adductcontaining uretdione groups, NCO number 13.3%, Degussa-Huls AG).

[0060] 2 Powder Coating Materials

[0061] 2.1 General Preparation Procedure

[0062] The comminuted products, i.e., acidic polyester,β-hydroxyalkylamide compound, polyisocyanate containing uretdionegroups, leveling agent, and catalysts if desired, are intimately mixedin an edge runner mill, together if appropriate with a white pigment,and the mixture is then homogenized in a twin-screw extruder (Berstorffequipment) up to a maximum of 140° C. After cooling, the extrudate isfractionated and ground with a pin mill to a particle size of <100 μm.The powder prepared in this manner is applied to degreased, optionallypretreated iron panels using an electrostatic powder spraying unit at 60kV and the panels are baked in a convection oven at temperatures rangingfrom 140 to 220° C.

[0063] The abbreviations in the table below have the followingsignifications:

[0064] CT=coat thickness in μm

[0065] EI=Erichsen indentation (DIN 53 156)

[0066] CH=crosshatch testing (DIN 53 151)

[0067] GG 60° angle=Gardner gloss measurement (ASTM-D 5233)

[0068] Imp. rev=impact reverse inch·lb

[0069] 2.2 Performance Testing TABLE 1 Pigmented powder coatingmaterials Example Formulation* 1 2 3 Comparative GRILESTA ® p 7617 60.3260.33 60.5 VESTAGON ® EP-HA 320 3.21 3.20 3.2 VESTAGON ® BF 1540 0.17 —— VESTAGON ® EP-BF 1310 — 0.17 — Notes: 35% by weight TiO₂ (whitepigment), 1.0% by weight Resiflow PV 88, 0.3% by weight benzoin Filmdata CT 75-80 58-72 61-69 CH 0 0 0 GG 60° angle 93 92 92 El >10 >10 >10Imp. rev. >160 >160 >160 Note no PH²⁾ no PH²⁾ C¹⁾, many PH²⁾ Curing:L200° C./10 minutes

[0070] The disclosure of German priority application Serial Number10008927.5 filed Feb. 25, 2000 is hereby incorporated by reference intothe present application.

[0071] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is,therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed herein

What is claimed as new and is intended to be secured by Letters Patentis:
 1. A transparent or pigmented powder coating material containingcrosslinking agents comprising hydroxyalkylamides and polyisocyanatescontaining uretdione groups, comprising: A) from 20 to 99% by weight ofat least one acidic polymer; B) from 1 to 25% by weight of at least oneβ-hydroxyalkylamide; C) from 0.05 to 10% by weight of at least onenonaromatic polyisocyanate which contains uretdione groups and has anNCO functionality≧1; D) from 0 to 50% by weight of at least one pigmentand/or inorganic filler; E) from 0 to 5% by weight of at least oneadjuvant or auxiliary.
 2. The powder coating material as claimed inclaim 1 , wherein said acidic polymer A) is a polyester resin having aglass transition temperature in the range from 30 to 85° C. and an acidnumber ranging from 20 to 80 mg KOH/g.
 3. The powder coating material asclaimed in claim 1 , wherein said acidic polymer A) is an acrylate resinhaving an acid number in the range from 10 to 250 mg KOH/g and a meltingpoint ranging from 60 to 160° C.
 4. The powder coating material asclaimed in claim 1 , wherein said acidic polymer A) is any desiredmixture of at least one polyester resin and at least one acrylate resin.5. The powder coating material as claimed in claim 1 , wherein saidβ-hydroxyalkylamide B) has formula I:

wherein: X is a chemical bond, hydrogen or a monovalent or polyvalentorganic group derived from saturated, unsaturated or aromatichydrocarbon groups and having 1-24 carbon atoms, or these radicalssubstituted by heteroatoms; R¹ is hydrogen, an alkyl, alkenyl, aryl oraralkyl radical having 1-24 carbon atoms, these radicals substituted byheteroatoms, or

each of R² independently being identical or different radicals andselected from the group consisting of hydrogen, an alkyl, aryl, aralkylor alkenyl radical having 1-24 carbon atoms, or these radicalssubstituted by heteroatoms; n is an integer from 1 to 10; m is aninteger from 0, 1 or 2; and n+m≧22.
 6. The powder coating material asclaimed in claim 1 , wherein the nonaromatic polyisocyanate containinguretdione groups, C) is selected from uretdiones of aliphatic,(cyclo)aliphatic and/or cycloaliphatic diisocyanates.
 7. The powdercoating material as claimed in claim 6 , wherein the nonaromaticpolyisocyanate containing uretdione groups, C) is selected from thegroup consisting of uretdiones of the diisocyanates isophoronediisocyanate, hexamethylene diisocyanate, 2-methylpentyl diisocyanate,2,2,4(2,4,4)-trimethylhexamethylene diisocyanate and mixtures thereof.8. The powder coating material as claimed in claim 6 , wherein thenonaromatic polyisocyanate containing uretdione groups, C) comprisespolyols as chain extenders.
 9. The powder coating material as claimed inclaim 8 , wherein the chain extenders are ethylene glycol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,3-methyl-1,5-pentanediol, 2,2,4(2,4,4)-trimethylhexanediol, neopentylglycol hydroxypivalate, trimethylolpropane, linear or branched,hydroxyl-containing polyesters, polycaprolactones or combinationsthereof.
 10. The powder coating material as claimed in claim 6 , whereinthe polyisocyanate C) carries, terminally, free or irreversibly blockedNCO groups or hydroxyl groups.
 11. The powder coating material asclaimed in claim 10 , wherein said irreversible blocking agent is amonohydric alcohol or a primary or secondary monoamine.
 12. The powdercoating material as claimed in claim 11 , wherein the polyisocyanate isblocked irreversibly with ethanol, n-butanol, 2-ethylhexanol,n-propylamine, n-butylamine, n-hexylamine, dibutylamine ordicyclohexylamine.
 13. The powder coating material as claimed in claim 1, wherein said at least one auxiliary and adjuvant is a leveling agent,degassing agent or catalyst.
 14. The powder coating material as claimedin claim 13 , which further comprises a catalyst in a concentrationranging from 0.03 to 1.0% by weight, based on the total amount of powdercoating material.
 15. The powder coating material as claimed in claim 14, wherein said catalyst is selected from the group consisting ofaluminum carboxylate salts, titanium carboxylate salts, aluminum oxides,zinc oxides, organotin compounds and combinations thereof.
 16. Thepowder coating material as claimed in claim 1 , wherein the ratio of theacidic polymer A) to the β-hydroxyalkylamide compound B) is selectedsuch that the ratio of carboxyl groups to hydroxyl groups ranges from0.6:1 to 1.6:1.
 17. The powder coating material as claimed in claim 1 ,wherein the ratio of the β-hydroxyalkylamide compound B) to thenonaromatic polyisocyanate containing uretdione groups, C), is selectedsuch that the ratio of hydroxyl groups to isocyanate groups ranges from0.5:1 to 2.0:1.
 18. A method of preparing a powder coating material,comprising: intimately mixing powdered materials of acidic polyester,β-hydroxyalkylamide, polyisocyanate containing uretdione groups andoptionally leveling agent, pigment, catalyst, adjuvant or combinationsthereof, homogenizing the mixture in a device at a temperature of up to140° C.; cooling the homogenized product; and fractionating and grindingthe homogenized product.
 19. A method of providing a substrate with apolymeric coating having excellent stability, comprising: spray coatingthe powder coating material of claim 1 onto said substrate; and thencuring the applied coating at a temperature of 140 to 220° C. over atime from 5 to 60 minutes.
 20. The method according to claim 19 ,wherein said spraying technique is electrostatic or tribostatic powderspraying.
 21. A method of providing a substrate with a polymeric coatinghaving excellent stability, comprising: coating by fluidized bedsintering the powder coating material of claim 1 onto said substrate;and then curing the applied coating at a temperature of 140 to 220° C.over a time from 5 to 60 minutes.